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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

MILES AND GRINNELL GLACIERS                           4S7
The remainder of the interior flat is mantled with outwash gravels supplied by three main glacial streams, one north and one south of the medial ridge and one between which flows into a subglacial channel beneath the ice of the terminal hill. These gravel surfaces are almost entirely barren. Practically all of the northern area of outwash is within the interior flat, although the stream cuts through the terminal hill in a narrow, steep-sided gorge which ends at Camp 52 on the railway. The southern and larger area of outwash gravels also extends outside the interior flat and is continuous with a large alluvial fan southeast of the glacier. This stream emerges from the interior flat in a constriction between the terminal hill and the stagnant southern lobe of the glacier. These outwash gravel fans slope at the rate of 400 to 900 feet to the mile.
Recent History of Grinnell Glacier. The first stage in the recent history of Grinnell Glacier of which record is preserved is a time of sufficient expansion so that lateral moraine terraces were built on the valley sides several hundred feet above the present glacier. With this augmented thickness the glacier would naturally have extended slightly farther into the Copper River valley than at present. Whether it then was a tributary of the trunk glacier of the Copper River valley or merely touched the distal portion of the more powerful Miles Glacier cannot be determined. The relationships of the stagnant moraine-covered termini of Grinnell and Miles Glaciers (PI. CLXV), make it clear that at some stage in the last period of great expansion these ice tongues, whose termini are still within 500 feet of each other, must have coalesced, the Copper River flowing beneath the ice or else maintaining a channel between the two ice tongues. This latter would be possible if the advances of Grinnell and Miles Glaciers were not synchronous, but the density and age of the vegetation on the two termini suggest that they were.
From 1891 to 1907 the Grinnell Glacier is not known to have changed appreciably. The photographs by Spencer and Moffit show no marked differences from the glacier of
1910.    In 1900 there was a forested terminal hill and an interior flat.    The age of trees on the terminal hill suggests that the last great expansion may have come shortly before 1892 but the observations of Hayes in 1891 and of Abercrombie and Allen in 1884 and 1885 furnish no details as to the previous conditions.
Mr. E. C. Hawkins informs us that in the autumn of 1907 there were great slides of morainic material from the terminal hill of Grinnell Glacier near Camp 52 and that a new stream appeared at that time.
A slight advance has been in progress from some time before 1907. This affects only the terminus of the cascading portion of the glacier. A comparison of the photograph by Spencer in 1900 with one by Moffit in 1907 indicates that a small area of the slope of bare rock in the middle of the cascading terminus was covered by the ice between 1900 and 1907. This advance may have continued ever since, or it may have ceased and a new period of advance started, for there was distinct minor advance from 1909 to 1911. Comparison of a 1909 photograph by E. A. Hegg, with the glacier terminus of 1910 showed marked advance. This continued at least up to June, 1911. It seemed to be at a very slow rate and in the two years of our own observations did not exceed one or two hundred feet. That there has been distinct forward movement and that it was in progress at the times of our visits was certain. In 1910 willows and alders up to 8 years of age were being overwhelmed by the advance of the ice margin. In June,
1911, the continuation of forward movement during the previous winter was shown by a series of snow arches, pushed up parallel to the edge of the glacier, and by the fact that,